1. Field of the Invention
The present invention pertains to processing gravity and magnetic data using vector and tensor data and more particularly to the inversion of gravity and magnetic data to locate possible hydrocarbon bearing zones near anomalies such as salt, igneous or magmatic formations.
2. Related Prior Art
Exploration for hydrocarbons in subsurface environments containing anomalous density variations has always presented problems for traditional seismic imaging techniques by concealing geologic structures beneath zones of anomalous density. The density by itself does not cause problems with seismic methods, but usually, large density contrasts are also associated with large velocity contrasts: this can cause severe ray-bending and problems with seismic processing. In this application, wherever reference is made to anomalous density, it is to be assumed that anomalous velocity is also present. Many methods for delineating the extent of the anomalous density zones exist.
U.S. Pat. No. 4,987,561, titled "Seismic Imaging of Steeply Dipping Geologic Interfaces," issued to David W. Bell, provides an excellent method for determining the side boundary of an anomalous density zone. This patent locates and identifies steeply dipping rock interfaces from seismic reflection data by first identifying select data that has characteristics indicating that the acoustic pulses that it represents have been reflected from a substantially horizontal or steeply dipping interface. This data is analyzed and processed to locate the steeply dipping interface. The processed data is displayed to illustrate the location and dip of the interface. This patent, while helping locate the boundaries, provides nothing to identify the subsurface formations on both sides of the boundary.
There have also been methods for identifying subsurface formations beneath anomalous zones using only seismic data to create a model and processing the data to identify formations in light of the model. By further processing acoustic seismic data, the original model is modified or adjusted to more closely approximate reality.
An example of further processing seismic data to improve a model is U.S. Pat. No. 4,964,103, titled "Three Dimensional Before Stack Depth Migration of Two Dimensional or Three Dimensional Data," issued to James H. Johnson. This patent provides a method of creating a three-dimensional model from two dimensional seismic data. This is done by providing a method of ray tracing to move before stack trace segments to their approximate three-dimensional position. The trace segments are scaled to depth, binned, stacked and compared to the seismic model. The model can then be changed to match the depth trace segments which will be stacked better, moved closer to their correct three-dimensional position and will compare better to the model. This patent uses a rather extensive seismic process to modify a seismic model that is not accurate. Seismic inversion schemes of the type disclosed in the Johnson patent can be quite accurate provided there is a good starting model for the velocity variations in the subsurface. This is hard to obtain in structurally complex areas, and for this reason, seismic inversion techniques used by themselves have a spotty record in geologically complex areas.
One type of geologic exploration data that has not been used extensively in the past is potential fields data, such as gravity and magnetic data, both vector and tensor data. Gravity gradiometry has been in existence for many years although the more sophisticated versions have been held as military secret until recently. The use of gravity data became acceptable in the late eighteen hundreds when measuring instruments with greater sensitivity were developed. Prior to this time, while gravity could be measured, variations in gravity caused by the effect of a large nearby object at one location, the gravity gradient and the gravity tensor, could not be reliably measured. The term gravity refers to a measurement of the earth's gravitational field g, the term gravity vector refers to the components of the gravitational field g.sub.x, g.sub.y and g.sub.z, while the term gravity tensor refers to the matrix of derivatives of the gravitational vector. This tensor has five independent components due to symmetry considerations. Similar meanings exist for the magnetic field, the magnetic field vector and the magnetic field tensor.
It has been known since the time of Sir Isaac Newton that bodies having mass exert a force on each other. The measurement of this force can identify large objects having a change in density even though the object is buried beneath the earth's surface or in other ways out of sight.
Exploration for hydrocarbons in subsurface environments containing anomalous density variations such as salt and evaporite formations, shale diapers, highly pressured sediments, and extrusive and intrusive igneous bodies create havoc on seismic imaging techniques by concealing geologic structures beneath zones of anomalous density and velocity. Such bodies, in addition to abnormal densities relative to the sediments in proximity, also have abnormal values of magnetic susceptibility. By utilizing gravity and/or magnetic field measurements along with a robust inversion process, these anomalous density zones can be modeled. The spatial resolution obtained from this process is normally much lower resolution than that obtained from acoustic seismic data. However, models obtained from gravity and magnetic data can provide a more accurate starting model. Using the potential methods data models as a starting point for two dimensional and three-dimensional seismic depth imaging greatly enhances the probability of mapping concealed geologic structures beneath the zones of anomalous density. The present invention increases the accuracy and resolution of gravity mapping by the use of seismic or other kinds of data to provide a good starting model for density variations above the anomalous regions.